When GIMP does not load on your mac even after you have been using it do this.
In a terminal:
The -F option prevents loading previous files and the -n creates a new instance
what I found was that somehow there is an instance that stays in memory until you reboot and if you don't want to reboot just run the following in your terminal.
open /Applications/GIMP.app -F -n
iPhone 6, iOS 12